Apparatus for transferring trays between a conveyor system and a stack

ABSTRACT

An apparatus for transferring baking trays and the like of ferromagnetic material between a tray conveyor system and a stack wherein an elevated tray conveyor system supports and advances articles laterally of a stack elevator which is operative to position the stack with the top tray at a level adjacent the tray conveyor, an endless type magnetic overhead conveyor overlies the elevator and the tray conveyor, and a tray transfer mechanism is provided for transferring trays between the overhead conveyor and the stack. The apparatus can be arranged to effect either a tray stacking or a tray unstacking operation.

United states Patent 1191 Breitbaeh 11 E Re. 28,551

[ Reissued Sept. 16, 1975 1 1 APPARATUS FOR TRANSFERRING TRAYS BETWEEN ACONVEYOR SYSTEM AND A STACK [76] Inventor: Johnnie J. Breitbach, 7Plymouth Ct., Normal, [11. 61761 [22] Filed: July 18, 1974 [21] Appl.No.: 489,823

Related U.S. Patent Documents Reissue of:

[64.] Patent No.: 3,770,143

lssued: Nov. 6, 1973 Appl. No.: 195,627 Filed: Nov. 4, 1971 [52] U.S. CI214/6 DS; 198/35; 214/8.5 A; 2l4/8.5 D; 271/18.1; 271/64 [51] Int. Cl.B65G 60/00 [58] Field of Search... 214/6 DS, 6 FS, 6 BA, 6 DK,'

214/6 H, 8.5 A, 8.5 C, 8.5 D, 8.5 R; 198/35;

[56] References Cited UNITED STATES PATENTS 2,015,809 10/1935 Moore214/6 DS 2,766,043 10/1956 Bucciconc 27l/l8.l 2,960,242 11/1960 Hcrr etal. 214/6 DS 3,122,229 2/1964 Engleson et a1. 214/6 BA x 3,224,75712/1965 Parke et a1 271 /18.1 3,342,350 9 1967 Seragnoli 214/6 BA x3,404,789 10 1968 Georgeff 61 61.. 2l4/8.5 D 3,409,149 11 1968 Graux271/18.1 3,517,835 6 1970 T6mp16..'. 214/8.5 D FOREIGN PATENTS ORAPPLICATIONS 1,546,148 10/1968 France... 214/6 DS 9 1964 Australia 214/6DS Primary ExaminerRobert J. Spar Assistant Examiner-Leslie J. PapernerI Attorney, Agent, or FirmMorsbach & Pillote 5 7 ABSTRACT An apparatusfor transferring baking trays and the like of ferromagnetic materialbetween a tray conveyor system and a stack wherein an elevated trayconveyor system supports and advances articles laterally of a stackelevator which is operative to position the stack with the top tray at alevel adjacent .the tray conveyor, an endless type-magnetic overheadconveyor overlies the elevator and the tray conveyor, and a traytransfer mechanism is provided for transferring trays between theoverhead conveyor and the stack. The apparatus can be arranged to effecteither a tray stacking or a tray unstacking operation.

23 Claims, 13 Drawing Figures Reissued Sept. 16, 1975 Re. 28,551

7 Sheets-Sheet 2 Reissued Sept. 16, 1975 7 Sheets-Sheet 5 Reissued Sept.16, 1915 7 Sheets-Sheet 4 Reissued Sept. 16, 1915 7 Sheets-Sheet 5 Mm m:

E St N Matter enclosed in heavy brackets I: 1 appears in the originalpatent but forms no part of this reissue specification; matter printedin italics indicates the additions made by reissue.

BACKGROUND OF THE INVENTION Many bread bakeries utilize a conveyorsystem for transporting the baking pans from the panner or doughdepositing station through the oven and then from the oven to thedepanner or bakery removal station. In a baking operation, a largenumber of baking pans are required to fill" the conveyor system whichtransports the pans from the panner through the ovens and to thedepanner and, in large baking runs which exceed the capacity of the pansnecessary to fill the conveyor system, it is desirable to recirculatethe pans from the depanner back to the panner. When starting a bakingoperation or when changing from one baking operation to anotherutilizing different size or shape pans, it is necessary to remove oneset of pans from the system and introduce a different set of pans intothe system. In addition, it is sometimes necessary, as when a back-up ofpans occurs somewhere in the baking system, to remove pans from theconveyor system until the cause of the back-up of pans has beencorrected.

It has heretofore been proposed as shown in the US. Pat. to Henry A.Heid No. 3,533,517, dated Oct. 13, 1970, to provide a pan stackingapparatus for removing pans from the pan conveyor system. In thatapparatus, the pans from the pan infeed conveyor first drop down onto apair of laterally spaced pan supports positioned above an elevator, andthe pan supports are thereafter moved laterally apart to allow the panto drop from the pan supports onto the top of the stack on the stackelevator. This two-step arrangement, wherein the pans first drop ontolateral pan supports and are thereafter dropped from the pan supportsonto the stack, increases the overall cycle time required to transferpans from the infeed conveyor to the stack. Further, the overall drop ofthe pans between the pan conveyor and the stack in such a two-steparrangement is relatively high and this increases the likelihood ofdamage to the pans during stacking. Moreover, this prior arrangement issuch as to require a separate by-pass conveyor and gating arrangementfor bypassing the pans around the stacker, in the event it is desired torecirculate the pans in the pan conveyor system past the stacker.

The pan stacking apparatus described above could not be utilized as anunstacking apparatus to feed pans into the pan conveyor system, and ithas heretofore been proposed, for example as shown in US. Pat. to Heideet al. No. 3,521,768, dated July 28, 1970, to provide a pan unstackingapparatus in which each stack of pans is moved from an upright positiononto an inclined stack feed conveyor, and the pans are then removed fromthe upper end of the inclined stack by upper and lower pan dischargeconveyors which engage the top pan in the stack. The time required fortilting each upright stack onto the inclined stack conveyor withoutdisrupting the stack, seriously limited the speed at which successivestacks of pans could be dispensed.

SUMMARY OF THE INVENTION The present invention relates to an apparatusfor transferring baking trays and the like of ferromagnetic materialbetween a tray conveyor system and a stack. The tray conveyor extendslaterally from a vertical stack elevator to underlie and support thetrays at a preselected level, and the stack elevator has a drivemechanism operative to maintain the top tray in the stack at a leveladjacent the level of the tray conveyor. An endless type overheadconveyor has its lower run spaced above the stack on the stack elevatorand above the tray conveyor and has magnetic means cooperable with thelower run for magnetically holding a tray against the lower run of theoverhead conveyor for movement therewith and a stack transfer means isprovided for transferring the trays directly between the overheadconveyor and the stack. It is frequently desirable, for example being inlong baking runs, to recirculate the baking trays in the tray conveyorsystem for rerun through a baking cycle and the magnetic overheadconveyor is accordingly advantageously arranged to span the inlet andoutlet tray conveyor sections at relatively opposite sides of the stackelevator so that it can also operate to transfer trays from the inlettray conveyor section to the outlet tray conveyor section forrecirculating the trays in the system.

In one aspect of the invention, the apparatus can be used to stack thebaking trays and the tray transfer means includes a tray ejectoroperable when a tray is moved along the magnetic overhead conveyor to aposition over the stack, for ejecting the tray from the overheadconveyor onto the stack. In order to enable selective recirculating ofthe trays from the inlet conveyor to an outlet conveyor, a movable stopis provided and is selectively movable between an operative positionstopping baking trays in a position over the elevator and an inoperativeposition out of the path of movement of the trays to allow bypassing ofthe trays from the inlet to the outlet tray conveyor.

In another aspect of the invention, the apparatus can be utilized as anunstacker to feed trays from a stack to the conveyor system. In suchapparatus, the tray transfer mechanism includes a magnetic meansengageable with the trays for picking up a tray from the top of thestack and for elevating the tray to the underside of the overheadconveyor for movement therewith.

An important object of this invention is to provide an apparatus fortransferring baking trays and the like between a tray conveyor systemand a stack which minimizes the overall cycle time required to transfera tray between the tray conveyor and the stack and which minimizesdamage to the tray during transfer between the tray conveyor and thestack.

Another object of this invention is to provide an apparatus fortransferring baking trays and the like between a tray conveyor systemand a stack, which is arranged to enable selective bypassing of thetrays in the tray conveyor system past the stack.

Still another object of this invention is to provide an apparatus fortransferring baking trays and the like between a tray conveyor systemand a stack which can be arranged for operation as a tray stackingapparatus and a tray unstacking apparatus.

These, together with other features and advantages of this inventionwill be more readily understood by ref erence to the followingdescription when taken in connection with the accompanying drawingswherein:

FIG. 1 is a side elevational view of a tray transfer apparatus for usein stacking trays;

FIG. 2 is an end elevational view of the ray transfer apparatus of FIG.I;

' FIG. 3 is a vertical sectional view taken on the plane 3-3 of FIG. 1;

FIG. 4 is a fragmentary sectional view taken on the plane 4-4 of FIG. 1and showing a portion of the tray elevator;

FIG. 5 is a top plan view of the tray transfer apparatus;

FIG. 6 is a fragmentary vertical sectional view taken on the plane 66 ofFIG. 5 and showing parts on a larger scale;

FIG. 7 is a fragmentary vertical sectional view showing a modified formof tray transfer apparatus for use in unstacking trays;

FIG. 8 is a schematic layout of a baking system employing the traytransfer apparatus of the present invention;

FIG. 9 is a diagrammatic view of a tray transfer apparatus arranged foroperation as a tray stacker;

FIG. 10 is a diagrammatic view of a tray transfer apparatus arranged foroperation as a tray unstacker;

FIG. 1 l is a schematic circuit diagram illustrating the electricalcontrols for use in adjusting the apparatus to accommodate trays ofdifferent width and height;

FIG. 12 is a schematic diagram of the electrical control circuit for atray transfer apparatus for use in stacking trays; and

FIG. 13 is a schematic diagram of the electrical control circuit for atray transfer apparatus for use in unstacking trays.

The apparatus of the present invention is generally adapted for use intray conveyor systems used in bakeries and the like to transfer bakingtrays between the tray conveyor and a stack. The tray transfer apparatuscan be arranged for stacking or unstacking baking pans of different sizeas well as baking pan lids and, as used herein, the term baking traysincludes baking pans and lids. As diagrammatically shown in FIG. 8, thetray conveyor system includes a conveyor section a for conveying traysfrom a dough depositing station or panner 21 to a lid applying station-or lidder 22, if lids are used. and a conveyor section 20b forconveyingthe trays from the lid applying station to an oven 23. The trays pass onconveyors through the oven and on a conveyor section 20c from the ovento a delidding apparatus 24 to remove the lids from the trays and aconveyor section 20d conveys the trays from the delidding apparatus tothe bread removal apparatus or depanner 25. When starting a bakingoperation, it is necessary to feed a large number of baking trayssequentially into the tray conveyor system and, in long baking runswhere the number of articles to be baked exceeds the capacity of thepans necessary to fill the system between the depositer and the remover,it is desirable to recirculate the pans from the depanner 25 back to thepanner 21. However, on the completion of a baking operation, it isnecessary to remove the pans from the system and, if a different size ortype of bakery product is to be produced requiring different pans, it isnecessary to introduce a different size or style of pan. Asdiagrammatically shown in FIG. 8, a tray conveyor section 20 is arrangedto convey trays from the depanner to a stacking apparatus 26 andaconveyor section 20f is arranged to convey trays from the stacker 26 toan unstacker 27, trays from the unstacker being conveyed by a furthertray conveyor section 20g to the panner 21. As also diagrammaticallyshown in FIG. 8, stacks of trays from the stacker 26 are conveyed by astack conveyor 30a to stack storage zones, conveniently in the form ofgravity type roller conveyors designated 3la-3le. A further stackconveyor 30b is conveniently provided to convey stacks of trays fromselected ones of the stack storage zones 31a-3le to the unstacker 27.While a pan stacker and a pan unstacker are diagrammatically shown inFIG. 8 for respectively removing and feeding baking pans into the panconveyor system, it will be apparent that a lid stacker and a lidunstacker can also be provided in the lid conveyor system 32 extendingbetween the delidder 24 and the lidder 22.

Referring now more specifically to FIG. 1, the tray transfer apparatusin general includes inlet and outlet tray conveyors 41 and 42 arrangedto underlie and support the baking trays and having their adjacent endsspaced apart at relatively opposite sides of a stack elevator mechanism43. A magnetic overhead conveyor 44 is spaced above the stack elevatorand above the adjacent ends of the inlet and outlet tray conveyors topick up a tray from the inlet conveyor 41 and move it laterally acrossthe top of the elevator. A tray transfer mechanism 45 is positionedabove the elevator for transferring trays from the overhead conveyor tothe stack on the elevator, and provision is made for rendering the traytransfer means inoperative so as to allow the baking trays to move fromthe inlet conveyor along the overhead conveyor to the outlet conveyorwithout passing to the stack. A stack conveyor 47 is provided adjacentthe lower end of the elevator to move stacks of baking trays laterallyof the elevator.

In the embodiment shown in FIGS. 1-6, the tray transfer apparatus isarranged to effect stacking of trays from the tray conveyor system, theapparatus with some modifications in construction and controls, beingalso arranged to effect unstacking of baking trays.

The stacking apparatus has a rigid frame structure including uprightmembers 51 adjacent the four corners of the machine; upper and lowerlengthwise extending frame members 52 and 53, and upper and lowercrosswise extending frame members 54 and 55 all rigidly interconnectedto form a rigid framework. The tray infeed conveyor 41 is preferably ofthe endless belt type including a belt 56 and a belt engaging roller 57supported as by a bracket 58 on the frame structure. The outlet conveyor42 is also conveniently of the end less belt type including a belt 61adapted to underlie and support baking trays, which belt is entrainedover a belt engaging roller 62 conveniently mounted as by a bracket 63on the frame structure.

The stack elevator 43 is of the endless chain type and includes pairs ofendless chains 65 and 65a disposed at relatively opposite sides of thearticle stack. One pair of chains 65 are disposed around upper and lowersprockets 65 and 67 nonrotatably secured to upper and lower shafts 68and 69 respectively and the other pair of chains 65a are disposed aroundupper and lower sprockets 66a, 67a mounted on shafts 68a and 69arespectively. The pairs of chains are mounted for adjustment relative toeach other in a direction laterally of the apparatus to accommodatebaking trays of different widths and, as best shown in FIGS. 2, 3 and 5,the ends of the upper shafts 68, 68a are rotatablymounted on followers.71, 71a that engage and are supported by oppositely threaded portions72, 72a of upper cross shafts 73. Shafts 73 are rotatably supported attheir ends by bearings 73a on the upper ends of the uprights. 51 so thatthe upper ends of the chains 65, 65a can be laterally adjusted towardand away from each otherby rotating the shaft 73 to adjust the followers71, 71a therealong. The lower shafts 69, 69a are similarly rotatablymounted at their ends on followers 74, 74a that engageand are supportedby oppositely threaded portions 75, 75a of lower cross shafts 76. Theends of the shafts 76 are supported by bearings 76a on the rigid frameuprights 51 so that, upon rotation of the shafts 76, the lower followers74, 74a are also adjusted toward and away from each other. Sprockets78a-78d areprovidecl on the pairs of upper and lower shafts 73and 76 andare drivingly interconnected by a chain 79. A reversible motor 81 has adrivesprocket that engages and drives the chain 79 to reversibly drivethe shafts 73 and 76 and thereby effect lateral adjustment of theelevator chains 65, 65a. Guide rails, conveniently in the form of angleirons 82, 82a are supported at their upper and lower ends by brackets 83that rotatably receive shafts 68, 68a and 69, 69a. The guide railsextend between the upper and lower shafts alongside respective ones ofthe elevator chains 65, 65a. Pairs of stack support members 85, 86 and85a, 86a are attached to the pairs of chains 65, 65a by suitablebrackets as best shown in FIG. 4, to extend horizontally between thechains of each pair, and followers 87, 88 are mounted on brackets 89 atthe ends of each of the stack support members to guidably engage therails 82, 82a to maintain the stack support members in a horizontalsupport position as shown in FIGS. 2 and 3, during movement of the stacksupport members along the rails. As shown, the followers 87, 88,arevertically and laterally offset from each other to engage the rear andfront sides of the guide rails at vertically spaced points therealong toprevent tilting of the stack support members 85, 86 and 85, 86a. Twostack supporot members are provided on each pair of chains 65, 65a andare located at equally spaced intervals along the respective elevatorchains so that only one of the stack support members on each chain isoperativeatany one time to support a stack of trays, the other stacksupport member being located along the return run of the elevatorchains.

The elevator drive mechanism is best shown in FIGS. 1 and 2 and includesa drive motor 91, which is connected through a drive chain 92 to theinput shaft 93a of a clutch and brake mechanism 93. The output shaft 93bof the clutch and brake mechanism is connected through a-drie chain94 tothe input shaft 95a of a speed reducer 95. The output shaft 95b of thespeed reducer is connected through a sprocket 96 and chain 97 to asprocket 98 on the upper elevator shaft 68, a movable chain tighteningsprocket 99 being provided and yieldably urged by a springin a directionto take up slack in the chain 97. The speed reducer output shaft 95b isalso connected through a sprocket 101 and chain 102 to an input sprocket103 on a gear box 104-.having an output sprocket 105 which is driven inthe reverse direction from its input sprocket. The output sprocket ofthe gear box 104 is connected through a chain 106 to a sprocket 17supported by a bearing bracket 107a and an elevator drive sprocket 96a.is nonrotatably con-- nectedto the sprocketl07 and drivesa chain97a-entrained overa sprocket 98a. on shaft68a. A second idler sprocket99a is provided'to take up slack in the chain 97a. The above-describedelevator drive is operative to drive the conveyor chains 65, a in timedrelation to each other and in relatively opposite directions, whileaccommodating lateral adjustment of the elevatorchains relative to eachother. In the preferred embodiment disclosed herein, the motor 91 isarranged to be continuously drivenand the elevator chains areintermittently driven under-.the control of, the clutchbrake mechanism93. v y

The overhead magnetic conveyor 44 extends between the' inletand outletconveyors 41 and 42 and overlies the elevator and,- advantageously, ismounted for vertical adjustment to accommodate baking traysof bearings1-16 in the longitudinal conveyor frame members 111. Screws 117threadedly engage followers in the upper ends of the upright framemembers 51 and the screws rotatably extend through blocks 119 on theconveyor frame members lll'and have thrust collars 121 thereon thatengage the. underside of the blocks 119 to support the'overhead conveyorframe on the screws. The overhead conveyor frame can thus be raised andlowered by rotating the screws 1'17 and sprockets. 122 are nonrotatablyseucred to the upper ends of the screws 117 and interconnected by achain 123 driven-as by a motor 124 (FIGS. 2 and 3).

As best shown in FIGS. 1 and 2, one of the overhead conveyor rollerssuch as the roller 114a is driven as by motor 125 mounted by a bracket126 on the conveyor frame, and connected through a drive chain 127 to asprocket on the shaft 114a. The belt 113 :for the overhead conveyor 44is formed of a nonmagnetic material such as woven cloth or rubber, and amagnetic rail assembly is provided for magnetically attracting bakingtrays of ferromagnetic material to the lower run of the belt 113. Asbest shown in FIGS. 5 and 6, themagnetic I the rails as by bolts 133.The rails 131 are formed of ferromagnetic'material and form pole piecesfor the magnets while guiding the lower. run of the conveyor therealong.As will be apparent, electromagnets could be utilized if desired.

Theoverhead conveyor is vertically adjusted relative to the inlet andoutlet conveyor 41 and 42 so that the 1 lower run of. the overheadconveyor is spaced thereabove a distance approximating the height of abaking tray .T and, preferably, the overhead conveyor overlaps the inletconveyor 41 as shown in FIG. 5 to assure pick up of the trays by theoverhead conveyor before they pass off the endof the inlet conveyor 41.Lateral tray guide rails 135 are provided for laterally guiding thetrays from the inlet conveyor onto the overhead eonveyor to laterallyposition the same with respect to the elevator. As shown-in FIGS.=3 and5, lateral tray guide rails are advantageously mounted by brackets 137on he followers"7l, 71a for the elevator and are posi- .ioned so as tobe vertically aligned with the inner sides )f the elevator chains 65,65a. Thus, the lateral guides are adjustable with the elevators toaccommodate pans if different width. 7

The tray transfer mechanism 45 is arranged to transer the trays betweenthe overhead conveyor 44 and the :levator 43. The tray transferapparatus shown in FIGS. [-6 is arranged for operation as the traystacker and he tray transfer mechanism 44 is accordingly arranged ejecttrays from the overhead magnetic conveyor to :ause the trays to droponto the stack. This transfer nechanism is best shown in FIGS. and 6 andincludes 1 rigid frame structure herein shown in the form of a air ofU-shaped side members 141 conveniently se- :ured to the'rails'131 of theoverhead conveyor to exend above the overhead conveyor 44. Tray engaginghoes 142 are disposed at relatively opposite sides of he overheadmagnetic conveyor 44 and have a length areferably corresponding to thelength of the trays T. The tray engaging shoes are mounted for verticalrecipocation and, in the embodiment shown, a pair of levers .43 arepivotally mounted intermediate their ends on ins 144 that extend betweenthe yokes 141. The outer :nds of the levers 143 are connected by crossrods 145 md links 146 to ears 142a on the'shoes 142 to raise and owerthe shoes in response to vertical swinging movenent of the levers 143.An operator, conveniently of he linear fluid actuated type including acylinder 147 .nd a rod 147a, is mounted on a cross member 149 that:xtends between the yokes 141. The piston rod 147b of he operator has apin l47c that extends through slots 43a in the inner ends of the lever143 to raise and ower the inner ends of the levers in response to movenent of the piston rod l47b. A tray stop finger l42b is rrovided at theinlet end of the shoes 142 ahd arranged o engage a succeeding tray onthe overhead conveyor 0 stop a succeeding tray on the overhead conveyorat location in advance of the elevator when the shoes removed downwardlyto eject a tray over the eleva- Apan stop is provided for stopping pansas they nove along the overhead conveyor, to position the ame above-theelevator 43. The pan stop is mounted or movement into and out of anoperative position nd, as shown, comprise a slide 154 that is mountedfor ertical movement in guideways 155 secured as by a mounting bracket156 to the main frame structure. An 'perator 157, conveniently of thelinear fluid actuated ype, is provided for raising andlowering the traystop nd; as best shown in FIG. 5, the cylinder portion of the perator157 is'secured to the bracket 156 and the rod ortion l57b is connectedto the tray stop to raise and Jwer the same.

The elevator drive mechanism is intermittently operted to maintain thetop of the stack of trays at a level lightly below the level of theinlet and outlet conveyors 1 and 42 and below the bottom of a traycarried by the verhead conveyor. A stack height sensing mechanism,onveniently in the form of a photoelectric cell 161 and iirror 162. ismounted so that the light beam from the hotoelectric cell 161 isinterrupted whenever the top f the stack is abovethe level of the inletand outlet 'ay conveyors. In addition, a second sensing device, [soconveniently in the form of a photoelectric unit 63 and mirror 164 ismounted" to sense the presence fa tray above the pan stop 154. Thephotoelectric unit 163 and mirror 164* are preferably arranged so thatthe light beam from the unit 163 extends diagonally across the locationof the pan'stop as best shown in F IG. 5 so that the light beam will beinterrupted by a tray somewhat in advance of the tray reaching aposition over the tray stop, and will remain interrupted until the traypasses beyond the tray stop.

A control circuit for operating the pan height and width adjustingmotors 124 and 81 is shown in FIG. 1 1. Motor 124 is of the reversibletype which can be selectively energized in a forward or reversedirection by multiple pole switches 171 and 172 each connected to themotor 124 through respective limit switches 173 and 174. For example,switch 171 is arranged to energize the motor in one direction to effectraising of the overhead'conveyor and limit switch 173 is arranged so asto open when the overhead conveyor reaches a preselected upper limit oftravel to prevent damage to the motor. Switch 172 is connected to themotor 124 to 0p erate it in a reverse direction so as to lower the overhead conveyor and limit switch 174 is arranged to be opened when theoverhead conveyor reaches a preselected lower limit of travel.Similarly, motor 81 for adjusting the width of the elevators includesmultiple pole switches 175 and 176 connected to the motor 81 so as torespectively operate the same in a forward and reverse direction andlimit switches 177 and 178 and connected in series'with the switches 175and 176 respectivelyto stop the motor 81 when the elevators reach apreselected inward limit of travel and a preselected outward limit oftravel respectively, to prevent damage to the motor. The stack conveyor47 is disposed adjacent the lower end of the elevator and isconveniently of the endless type including a belt 181 entrained overrollers 182 and 183 rotatably mounted as by brackets 184 and 185 on theframe and driven by a motor 186. The upper run of thestack conveyor isdisposed at a level slightly above the axes of the lower elevatorsprockets as'best shown in FIG. 3 so as to engage and convey a stacklaterally when the stack is lowered.

A control circuit suitable for operating the apparatus as a tray stackeris shown in FIG. 12. The overhead conveyor drive motor 125, the elevatordrive motor 91 and the stack conveyor drive motor 186 are connected inparallel and through contacts 191a of a motor start relay 191 to asource of electrical power S. The power source is also connected througha transformer 192, the secondary of which is connected to low voltageconductors 193 and 194. A normally closed stop switch 195 and a normallyopen start switch 196 are connected in series with the motor start relay191 which is operative, when energized, to close the relay contacts 191ato start the motors 125, 91 and 186. Relay 191 also closes contacts 19lbconnected in parallel with the start switch 196 to establish a holdingcircuit to maintain the relay energized when the start switch is released. An indicator lamp 197 is connected in parallel with the relay191 to indicate when the latter is ener gized. The aforementionedholding circuit also controls the application of power to conductor 193aand photoelectric units 161 and 163 are connected as by conductors 199to the power conductors 193a and 194 so as to be energized when the mainmotor control relay 191 is energized.

Fluid is reversibly supplied to the operator 147 of the article transfermechanism under the control of a valve 120 (FIGS. 2 and 3). The valve201 is conveniently of the electrically operated type havingelectroresponsive operators 201a and lb arranged so that, when oneoperator such as 201a is energized, the valve is moved to one positionto supply fluid in one direction to the operator 147, and the valve thenremains in that posi tion until the other electroresponsive operator20lb is energized to return the valve to its initial position andreverse the application of fluid pressure to the operator 147. Anormally open pan switch 202 is provided on the tray stop 154 and isarranged to be moved to a closed position when engaged by a tray. Limitswitches 203a and 203b are mounted as diagrammatically shown in FIG. 9so as to sense when the transfer mechanism is respectively in its raisedposition shown in FIG. 1 and a lowered position. Switches 203a and 203bare of the normally open type and switch 203a is closed when thetransfer mechanism is in its raised position and 203b is closed when thetransfer mechanism moves downwardly to its tray eject position. Switches203a and 203b are connected in series with their respective valveoperators 201a and 20lb and are connected through the tray sensingswitch 202 and normally closed relay contacts 207C to the powerconductor 193a. When a tray closes the switch 202, it establishes acircuit through limit switch 203a (closed when the shoes are in theirraised position) to energize the valve operator 201a and apply fluidpressure to the actuator 147 to move the tray engaging shoes 142downwardly. When the tray engaging shoes begin to move downwardly, theswitch 203a is opened and, when the shoes move to a level slightly belowthe lower run of the over head conveyor, they eject the pan from theoverhead conveyor. Before the tray sensing switch 202 is opened, theshoes move downwardly to a position closing switch 203b to energize theoperator 20lb and thereby move the actuator back to its raised position.The tray sensing switch 202 opens after the tray drops from the overheadconveyor and before the tray engaging shoes are retracted to a positionclosing switch 203a to prevent recycling of the transfer mechanism 45.

The clutch-brake mechanism 93 has an electroresponsive operator 93cwhich is operative, when energized, to engage the clutch and disengagethe brake and, conversely, operative when deenergized to disengage theclutch and engage the brake to stop the elevator drive.Electroresponsive operator 93c is connected to the power conductor 193athrough a normally open pan sensing switch 202a which is arranged to beoperated in unison with the switch 202. Normally open relay contacts207b are connected in parallel with the switch 202a and normally openphotoelectric relay contacts 161a are also connected in parallel withthe switch 202a. When a tray engages the stop 154, it closes switch 202ato energize the operator 93c for the elevator clutch to start theelevator drive and began lowering of the stack. Normally openphotoelectric relay contacts 161a are operated by the stack heightsensing photoelectric unit 161 and are closed when the tray is droppedonto the stack and interrupts the light beam from the photoelectric unit161 so that the elevator clutch remains energized until the stack haslowered sufficient to reestablish the light beam to photoelectric unit161. With this arrangement, the elevator drive is intermittentlyoperated to lower the stack of trays on the elevator to position the topof the stack at the level adjacent to but slightly below the level ofthe inlet and outlet tray conveyors 41 and 42. When a predeterminednumber of trays have accumulated in the stack, the tray elevator closesa normally open limit switch-205 conveniently positioned as shown inFIGS. 3 and 9 to sense when one of the stack supports reaches apreselected lower position corresponding to a full stack of trays on theelevator. Switch 205 is connected in a series circuit with a normallyclosed limit switch 206 to a control relay 207 to energize the con- 7trol relay when the switch 205 is closed. Relay 207 is arranged to closenormally open relay contacts 207a connected in parallel with the limitswitch 205 to establish a holding circuit for the control relay andrelay 207 also controls the aforementioned contacts 207b connected inparallel with the limit switch 202a and photoelectric relay contacts161a so as to establish a circuit to the operator 93c for the elevatorclutch. Thus, when the limit switch 205is operated to its closedposition in response to accumulation of a predetermined number of traysin a stack, the elevator clutch is energized under the control of relay207 and contacts 207b and remains energized to lower the stack of traysonto the stack conveyor and to bring the other stack support intoposition to support a tray at a level adjacent the inlet conveyorl Limitswitch 206 is conveniently positioned as shown in FIGS. 3 and 9 to sensewhen the stack support has moved into a position to receive the firsttray in the stack and to support the same as the aforementioned leve.Switch 206 is then opened to break the circuit to the control relay 207and deenergize the clutch actuator 93c. A selectively operable manualswitch 209 is conveniently connected in parallel with the limit switch205 to enable selective dumping of partial stacks of trays. Controlrelay 207 is also operative, when energized, to open the aforementionedrelay contacts 207e, to prevent operation of the tray transfer mechanismduring indexing of the stack elevator.

In the long baking runs it is desirable to recirculate the pans from thedepanner 25 back to the panner 21 without passing the pans through thestacking and unstacking apparatus. The overhead magnetic conveyor 44 isarranged to transfer trays from the inlet conveyor 41 to the outletconveyor 42 and provision is made for retracting the tray stop 154 toallow bypassing of the stacking apparatus. The inlet and outletconveyors are driven as by motors (not shown) at linear speedsapproximately equal to the linear speed of the overhead conveyor 44 sothat there will be a smooth transfer of trays from the inlet conveyor tothe overhead conveyor and from the overhead conveyor to the outletconveyor. The fluid actuator 157 for the tray stop is operated under thecontrol of a valve 212 (FIG. 9) having an electroresponsive operator212a. The valve 212 is of a type which is normally positioned to applyfluid pressure to the operator 157 to hold the tray stop in its raisedposition and the valve is operative, when the electroresponsive operator212a is energized, to move the tray stop to its lower position to permitstraightthrough feed of the baking trays from the inlet conveyor alongthe overhead conveyor to the outlet conveyor. The tray stop operator212a and a relay 218 are connected in parallel with each other and inseries with a normally open mode switch 214, a normally open remotesignal switch 215a, and normally closed relay contacts 163a operated byphotoelectric relay 163. If the mode switch 214 is moved to its openposition, valve actuator 212a will be deenergized and the valve 212 willmove to a position raising the tray stop to start stacking of trays.However. when the mode switch 215 is moved to its closed position forautomatic operation, stacking of trays will be under the control of theremote signal switch 215a. Switch 215a is normally closed to energizethe tray stop valve operator 212a, when relay contacts 163a are closed.Relay contacts 163a, as noted above, are normally closed and are openedonly when the light beam of photoelectric unit 163 is inter ruptedindicating the presence of a tray over the elevator and tray stop. Theremote signal switch 215a is preferably operated by a photoelectric unit215 positioned as diagrammatically shown in FIG. 8 to sense a back-up orjam-up of trays in the conveyor system f downstream of the articlestacking apparatus and the remote photoelectric unit 215 is arranged toopen the relay contacts 215 whenever a back-up of trays occurs in theconveyor. The photoelectric unit 215 must be arranged to prevent openingof the contacts 215a due to the intermittent interruption of the lightbeam caused by trays passing the unit 215, and to open the relaycontacts 215a only when the light beam remains interrupted for a timeinterval in excess of that normally caused by a passing tray, and whichextended light beam interruption would indicate a jam-up of trays in theconveyor system. For example the photoelectric unit may be of a typewhich delays opening of contacts 215a for a predetermined time intervalafter the light beam is interrupted, which interval is sufficient toallow passage of an individual tray past the photoelectric unit. If thelight beam is reestablished in that time interval, the photoelectricunit will continue to hold the contacts 215a open to allow trays tobypass the stacker. However, if the light beam is interrupted for a timegreater than that interval, then the contacts 215a will be opened tocommence stacking of trays. Thus, when the mode switch 214 is moved toits closed or automatic position, and relay contacts 215a are closedindicating the absence of a jam-up in the outlet conveyor system, andrelay contacts 163a are closed indicating the absence of a tray abovethe elevator and adjacent the tray stop, then valve actuator 212a willbe energized to lower the tray stop. Relay 218 will also be energized toclose contacts 218a to maintain actuator 212a energized as contacts 163aopen and close due to the trays moving past the photoelectric unit 163.

It is also necessary to prevent raising of the tray stop when a tray isin position over the tray stop and for this purpose, normally open relaycontacts 218b are connected in series with normally open relay contacts163b to the valve actuator 212a and relay 218. Relay contacts l28b areclosed when the relay 218 is energized and relay contacts 163b areoperated under the control of photoelectric unit 163, so as to be closedwhen the light beam to the unit 163 is interrupted and opened when thelight beam is again established, indicating the absence of a tray overthe tray stop. Thus, when relay contacts 215a are opened indicating ajamup of trays in the outlet tray conveyor, or if the mode switch 214 isopened, the holding circuit including relay contacts l63b and 218b willmaintain the tray stop actuator 212a energized until photoelectric relay163 opens contacts 163b indicating that the trays have cleared the areaover the tray stop. The tray stop can then move to its raised positionto begin a stacking operationv The tray transfer apparatus can also bearranged for operation as a tray unstacker to feed trays from a stackinto the tray conveyor system. The construction of the tray unstacker issubstantially the same as the stacker and like numerals are used todesignate corresponding parts. The tray unstacker does not receive atray stop and the tray stop can accordingly be omitted. In addition somemodification of the tray transfer mechanism 45 and the circuitry forcontrolling the apparatus is necessary for operation as an unstacker. Asshown in HS. 7, the tray transfer mechanism 45' is provided with trayengaging shoes of ferromagnetic material 142' having magnetic meansassociated therewith and arranged to magnetically attract aferromagnetic tray to the shoes to enable the shoes to pick up a panfrom the top of the stack and raise it to the underside of the overheadmagnetic conveyor 44. in the preferred embodiment, permanent magnets areutilized, it being understood that electromagnets could be utilizedalong the shoes 142'. In addition, the operating mechanism for raisingand lowering the shoes is arranged to move the shoes through arelatively greater distance than in the stacker and suffieient to engagethe top tray in the stack and raise the tray to the underside of theoverhead conveyor. As shown in FIG. 7, the shoes 142' are verticallyraised and lowered by a pair of crank arms 143' connected by pins 145'to upper ends of links 146, the lower ends of which links are pivotallyconnected to ears 142a on the shoes 142. The cranks are rotated inunison with each other by a chain 144' entrained around sprockets 144awhich are nonrotatably connected to respective ones of the cranks 143'so that the shoes remain horizontal during raising and lowering. Thesprockets and cranks are rotatably mounted on support members 149conveniently supported by brackets 141' on the rails 131 of the overheadconveyor. A linear actuator including a cylinder 147 attached to thesupport members 149, and a rod 147a, is provided for operating thetransfer mechanism. The rod 147a is connected at 147b to the chain 144'and the rod has sufficient travel to cause the cranks to move through acomplete revolution when the rod is moved in one direction and to movethrough a second revolution when the rod is moved in the oppositedirection.

Vertical guide rods 150 are rigidly attached to the shoes 142 andslidably extend through fixed guides 151' to inhibit lengthwise shiftingof the shoes during vertical movement of the latter. A valve 201 isprovided for reversibly supplying fluid such as air under pressure tothe cylinder 147 and the valve has electroresponsive actuators 201a and201b. Limit switch 203' is provided for sensing travel of the rod 147aof the linear actuator 147'.

A modified control circuit shown in FIG. 13 is utilized to operate thetray unstacking apparatus. in the unstacking apparatus, the top beltdrive motor 125 is energized under the control of relay contacts 301acontrolled by relay 301; the elevator drive motor 91 is reversiblyenergized under the control of relay contacts 302a and 303a respectivelyoperated by forward and reverse relays 302 and 303, and the stackconveyor drive motor 186 is energized under the control of contacts 304aoperated by relay 304. For reasons pointed out hereinafter, the inlettray conveyor 41 of the stacker is driven at approximately the samespeed as the overhead conveyor 44 by a drive motor 120 also energizedunder-the control of relay contacts 301a that control the overheadconveyor drive motor 125. The

control circuit includes power conductors'306 and 307 the off positionshown in FIG. 13, to interrupt application of power to the relay 301 andwhich is operative in its on position to close switch 309a to energizethe relay 301. The mode switch is also operative in its auto position toopen contacts 309a and close contacts 309b which last-mentioned contactsare connected in a series circuit with normally closed relay contacts311a.

The forward relay 302 for energizing the elevator drive motor 81 isconnected in series with normally closed relay contacts 303a controlledby the reverse relay 303, normally open manually operable start switch313, normally closed manually operable stop switch'314, and an elevatormode switch 315 having one set of contacts 315a which are closed whenthe switch 315 is in one position and a second set of contacts 315bwhich are closed when the switch is in the other position. Thus, whenswitch 315 is in its unstack position closing contacts 315a, and whenstart switch 313 is closed, the forward relay 302 is energized andcloses normally open relay contacts 302a in parallel with the startswitch to establish a holding circuit. The forward relay will thusremainenergized until either the stop switch 314 is opened or the reverserelay 303 is energized to open relay contacts 303a or the mode switch315 'is moved to a position opening contacts 315a and closing contacts315b. An indicator light 316 is conveniently connected in parallel withthe relay 302 to indicate when the elevator motor is energized in itsforward direction. The elevator motor is normally energized in itsforward direction during unstacking. However, it is sometimes desirableto discharge a partial stack from the unstacking apparatus and, for thispurpose, the reverse relay 303 is on nected in a series circuit withnormally closed relay contacts 302b controlled by forward relay 302, anormally open manually operable switch 318 and the switch contacts 3151)of the mode switch 315. When the mode switch is in its lower positionclosing contacts 315b, and the manually operable switch 318 is closed,the reverse relay 303 is energized to close contacts 303a andoperate'the elevator in a reverse direction.

The elevator forward drive relay 302 also is operative, when energized,to close contacts 302c to apply power from conductor 306 to conductor306a. The photoelectric unit 161 for sensing the level of the top of thestack, the photoelectric unit 163' and the photoelectric unit 311 areconnected so as to be energized when the relay contacts 302C are closed.Photoelectric unit l63"is arranged with its mirror 163a, asdiagrammatically shown in FlG. 10,to sense the presence of a tray on theoverhead conveyor 44 from a point in advance of the inlet side of theelevator 43 to a point after the outlet side of the elevator. Thephotoelectric unit through contacts 321a of a mode switch 321, normallyopen limit switch 322, and relay contacts 323a controlled by a felay 323to the power conductor 306a. Limit switch 322 is normally open and ispositioned as diagrammatically shown in FIG. 10', so as to be energizedto its closed position when the stack on the elevator is substantiallydepleted and the elevator has moved so that the tray supports 85, a areat a level above the level of the top of a succeeding stack. The modeswitch 321 is normally in the position shown in FIG. 13 closing contacts321a and opening contacts 321b so that, when the switch 322 is closed,relay 304 is energized to close motor control relay contacts 304a toenergize the stack drive motor 186 to drive the stack conveyor in thedirection indicated by the arrows in FIG. 10 and advance a stack fromconveyor 30b toward the unstacker. As diagrammatically shown in FIG. 10,the stack conveyor section 30b is conveniently driven by a powertake-off 47a from stack conveyor 47. Relay 304 also closes relaycontacts 304b connected in parallel with the limit switch 322 toestablish a holding circuit to maintain the motor 186 energized, untilthe relay contacts 323a are opened. The stack conveyor 30b, 47 will thenadvance I a succeeding stack into a position between the chains of theelevator and, when the stack is in position, it closes a limit switch326 positioned as diagrammatically shown in FIG. 10 to sense when astack is in proper position on the stack conveyor 47 between the chainsof the elevator 43. Closing limit switch 326 energizes relay 323 to openthe normally closed relay contacts 323a and stop the stack conveyordrive motor 186.The mode switch 321 is operable in a second positionthereof to open contacts 321a and close contacts 32Ib to directlyenergize the'motor control relay 304" to drive the stack conveyor 47 inthe direction indicated by the arrow in FIG; 10 and move a stack out ofa position below the elevator onto a stack outlet conveyor 47b. 1

Limit switch 203 is arranged as diagrammatically shown in FIGS. 7 and 10to be actuated when the rod 147a of the actuator 147' is retracted adistance to drive the crank amis 143' through one complete revolution.Limit switch 203' includes normally closed switch 203a connected inseries with the valve operator 201a and a normally open switch 203b'connected in series with valve operator 201b. Limit switches 201a and20lb' are normally in the position shown in FIG. 13 and are respectivelymoved to their open and closed positions when the piston rod isretracted a distance to drive thecranks 143 through a complete rev- Iolution. Valve 201' is similar to the previously described valve 201 andis of a type which is arranged to controlled by photoelectric relay 163.and normally closed relay contacts 161a controlled by photoelectricrelay unit 161 As previously described, photoelectric relay unit 161 isarranged to sense the top of the stack on the elevator and operates toclose relay contacts 161a when the top of the stack is slightly belowthe level sensed by the unit 161'. Photoelectric unit 163' senses thepresence or absence at any location of a tray on the overhead conveyor44 betweena location substanti'ally in advance of the elevator to alocation adja- :ent the outlet end of the overhead conveyor, and theclay contacts 163a are open when the tray is in that irea and thecontacts are closed in the absence of a tray n that area. Accordingly,if the top of the stack is at the )roper level sensed by photoelectricunit 161', and in he absence of a tray on the overhead conveyor sensed1y the photoelectric unit'l63, operation of the tray ransfer actuators201a and 201b will be controlled y manual switch 324. When switch 324 isclosed, the 'alve actuators 203a and 203b' will be cyclically operltedto pick up a tray from the top of the stack and ransfer it to theoverhead magnetic conveyor. In paricular, when a tray moves off theoverhead conveyor 0 the outlet conveyor 42 and photoelectric unit 163'loses relay contacts 163a, valve actuator 201a is en- :rgized to retractthe piston rod 147a to thereby rotate he cranks 143' through onerevolution and move the hoes 142a first down to pick up a tray from thestack .nd then up to transfer the tray to the overhead coneyor. When thepiston rod reaches the end of its reraction, it operates limit switch203 to open switch 03a and close switch 203b'. However, as the shoes 42pick up a tray from the stack, the tray interrupts he light beam ofphotoelectric unit 163 and opens ontacts 163a to prevent energization ofvalve actuaor 203b' until the overhead conveyor has moved the rayawayfrom a position over the stack. When the tray 'asses the light beam fromunit 163, contacts 163a re again closed to operate the shoes 142throughanther tray pick-up cycle. I

The electroresponsive operator 93c for the elevator lutch brakemechanism is energized under the control f relay contacts l6lb' operatedby the stack level phonele ctric sensing unit 161 to drive the elevatoruplardly to maintain the top tray in the stack at the se acted level.Relay contacts l6lb' are open when the ght beam is interruptedindicating that the stack is bove the desired level, and are closed whenthe light eam is reestablished indicating that the stack is at orlightly below the desired level. Relay contacts l6lb' re connected inseries with normally closed relay ontacts 335a operated by relay 335 andnormally losed relay contacts 303b operated by the aforedesribedelevator reverse relay 303' to energize the lutch-brake 93 when thestack is below the desired :vel and to deenergize the clutch-brake whenthe stack caches the desired level. Relay contacts 303b are normallyclosed and are opened only when the relay 303 i energized to drive theelevator in a reverse direction. Lelay contacts 335a are normally closedand are enerized to their open position when relay 335 is energized nderthe control of limit switch 336. Limit switch 336 L normally open and ispositioned as diagrammatically iown in FIG. 10 so as to be closed whenthe operative :ack support member is moved to a position slightly bovethe position in which it supports the last stack at 1e desired level.When switch 336 is closed and ener-. izes relay 335, relay 335 openscontacts 335a to preent operation of the elevator under the control ofthe ho toelectric unit 161', and closes contacts 335b conected in serieswith normally open relay contacts 323b as to energize the elevatorclutchbrake when both antacts 323b and 335b are closed. Relay 335 alsooprates, when energized, to close normally open antacts 335C connectedin series circuit with normally pen contacts 323C also controlled byrelay 323 so as I establish a holding circuitfor the relay 335 when thelimit switch 336 is closed. This holding circuit maintains the relay 335energized after the limit switch 336 opens and until the stack positionsensing switch 326 opens in response to raising of a stack by the stackelevator off the stack conveyor. At that time, relay 335 is deenergizedand closes contacts 335a so that continued movement of the elevator iscontrolled by the stack height sensing photoelectric unit 161 Theelevator reverse relay 303 for operating the stack elevator in reversealso operates to close normally open contacts 303C connected in serieswith the elevator clutch-brake actuator 93c so that, when the elevatordrive motor is operated in its reverse direction, the clutch-brake isalso energized to move the elevator in a reverse direction to lower thesame and discharge a partial stack onto the stack conveyor.

OPERATION From the foregoing it is thought that the construction andoperation of the apparatus for transferring pans between a stack and thetray conveyors will be readily understood. The same basic apparatus canbe utilized for both a tray stacker and a tray unstacker. The magneticoverhead conveyor is arranged so that it will pick up trays from theincoming tray conveyor and move them across the elevator to the outletconveyor. This enables bypassing of the stacker or unstacker when it isdesired to recirculate the trays in the tray conveyor system.

When utilized as a stacking apparatus, the tray stop can be selectivelymoved into and out of position stopping a tray in position above theelevator and the switch 202 on the tray stop automatically operates thetray transfer mechanism to discharge a tray from the overhead conveyorto the stack. The elevator drive mechanism operates in response to thestack level to lower the stack and maintain the top of the stack at alevel closely adjacent the level of the trays on the inlet and outletconveyors. With this arrangement, the trays undergo only a relativelyshort drop determined by the height of the trays and, moreover,transferring of the trays from the overhead conveyor to the stack iseffected in a single step so as to minimize the overall time required tostack each tray. Moreover, since the trays only drop through arelatively short distance determined by the height of the trays, thestack can accumulate to substantially the full height of the elevatorbefore it is necessary to discharge the stack onto the stack conveyor.This not only increases the overall capacity of the apparatus for agiven overall height, but additionally minimizes the transfer timenecessary to move a succeeding stack support into position after acompleted stack is discharged. The tray stop can be moved to its raisedposition by moving manually operable mode switch 214 to its openposition to effect stacking of all trays that enter the stackingapparatus. Alternatively, when mode switch 214 is in its closedposition, the tray stop will normally be in its lower position so thatthe trays enter ing the stacking apparatus will pass along the overheadconveyor 44 to the outlet conveyor 42 of the stacker. However, when thephotoelectric unit 215 senses a back-up of trays on the outlet conveyor,it opens relay contacts 215a to effect raising of the tray stop tocommence stacking until the back-up of trays on the outlet conveyor hascleared.

When utilized as an unstacker, the magnetic pick-up shoes 142' arecyclically operated to pick up trays from the top of the stack andtransfer the trays to the overhead conveyor 44 for passage to the outletconveyor 42. The photoelectric unit 163 will sense ifa tray is en teringthe unit on the inlet tray conveyor 41 and will prevent operation of theunstacker until the tray has cleared the unstacker so as toautomatically bypass trays from the inlet conveyor to the outletconveyor. Switch 324 can be opened to stop operation of the unstacker.However, when switch 324 is closed, the oper ation of the unstacker willautomatically be stopped if the photoelectric unit 311 senses a back-upof pans on the conveyor section 20g. As described above, photoelectricunit 311 operates to open relay contacts 311a when a back-up of pans issensed and this operates to stop the overhead magnetic conveyor 44 andthe inlet conveyor 41 (corresponding to conveyor section 20f in FIG. 8).When the overhead conveyor 44 of the un stacker is stopped, it cannotremove pans from above the elevator so that operation of the traytransfer mechanism 45 of the unstacker is also stopped. In addition,stopping of conveyor section 20f will also effect operation of thephotoelectric unit 215 of the stacker to start stacking of traysentering the stacker.

While preferred embodiments of the invention are herein specificallydisclosed, it will be apparent to those skilled in the art thatmodifications can be made within the scope of the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An apparatus for use in a recirculating loop baking tray conveyorsystem for transferring baking trays and the like of ferromagneticmaterial between a tray conveyor system and a stack comprising:

said recirculating loop baking tray conveyor system including inlet andoutlet tray conveyor sections each adapted to underlie and supportbaking trays at a selected level and having adjacent ends spaced apart adistance substantially greater than the length of a tray,

stack elevator means for supporting an upright stack of trays betweensaid adjacent ends of said inlet and outlet tray conveyor sections, saidstack elevator means including laterally spaced endless chains extendingvertically downwardly from adjacent said preselected level at relativelyopposite sides of the path of travel of the trays on the overheadconveyor means, said laterally spaced chains having stack supports atspaced locations therealong adapted to underlie and support thelowermost tray in a stack disposed between the chains, means for sensingthe level of the top of the stack of trays between said chains, andelevator I: dirve drive means responsive to said stack level sensingmeans for driving said chains to position the top tray in the stack at alevel slightly below said selected level of said inlet and outlet trayconveyor sections,

said baking tray conveyor system including an endless type overheadconveyor means having a generally horizontal lower run dimensioned to atleast span the space between the inlet and outlet conveyor sections,magnetic means cooperable with said lower run for magnetically holdingthe top of a ferromagnetic baking tray against the lower run formovement therewith, and means mounting said overhead conveyor means forvertical adjustment relative to said inlet and outlet conveyor sectionsand to said endless chains of said stack elevator means to position saidoverhead conveyor means with its lower run at a level spaced above theselected level of the inlet and outlet tray conveyor sections a distancecorresponding to the height of the baking trays to allow trays to movefrom the inlet conveyor section along the lower run of the overheadconveyor and onto the outlet section for recirculating the trays,

and tray transfer means intermittently operable to transfer traysbetween the overhead conveyor means and the stack.

2. An apparatus according to claim 1 including means operative inresponse to advance to a tray along said overhead conveyor means to aposition above said elevator means for operating said transfer means tomove a tray from the overhead conveyor means to a stack on said elevatormeans.

3. An apparatus according to claim 1 including stop means mounted formovement between an operative position in the path of movement of traysalong said overhead conveyor means and an inoperative position out ofthe path of movement of said trays, said stop means in said operativeposition thereof being arranged to stop trays on said overhead conveyormeans above said elevator means.

4. An apparatus according to claim 3 including means operative when atray engages said stop means for operating said transfer means to move atray from the overhead conveyor means to a stack on said elevator means.

5. An apparatus according to claim 3 including selectively operablemeans for moving said stop means to its operative position, means forsensing the presence of a tray in the path of movement of said stopmeans and for preventing movement of said stop means to its operativeposition until the tray has ,cleared the path of movement of the" stopmeans.

6. An apparatus according to claim 5 wherein said selectively operablemeans for moving said stop means to its operative position includesmeans for sensing a back-up of traysat a selected location along theoutlet conveyor I: means section in the tray conveyor system.

7. An apparatus according to claim 1 including means operativein'response to advance of a tray along said overhead conveyor means to aposition above said elevator means for operating said transfer means todischarge a tray from said overhead conveyor onto a stack on saidelevator means, stack conveyor means adjacent the lower end of saidelevator means for conveying a stack of trays laterally away from theelevator means, and means operative when a preselected number of trayshas accumulated in the stack for operating said elevator means to lowerthe stack onto said stack conveyor means.

8. An I: appratus apparatus according to claim 1 wherein said transfermeans includes an elongated shoe engageable with the top of the trays onsaid overhead conveyor means for disengaging the trays from saidoverhead conveyor means.

9. An apparatus according to claim 1 wherein said tray transfer meansincludes a magnetic pickup head mounted for movement between a lowerposition engaging a tray at the upper end of the stack and a raisedposition to transfer the top tray from the stack to the overheadconveyor means.

10. An apparatus according to claim 9 including means for sensing thepresence or absence of a tray on the overhead conveyor means and forpreventing operation of said transfer means until a tray on the overheadconveyor means has cleared a position above the elevator.

11. An apparatus according to claim 9 including stack conveyor meansadjacent the lower end of said elevator means for advancing a stack ofarticles to the elevator means, means for securing depletion of thestack on the elevator means to a preselected minimum for operating saidstack conveyor means to advance a succeeding stack of trays to theelevator means, and means operative when the stack of articles on theelevator means is exhausted for driving the elevator means to pick upsaid succeeding stack of trays.

12. An apparatus according to claim 11 including means for sensing thepresence or absence of a tray on said overhead conveyor means, and meansfor preventing operation of said tray transfer means when a tray is onthe overhead conveyor means above the elevator means.

13. An apparatus according to claim including means for sensing aback-up of trays on the outlet conveyor means, and means operated bysaid lastmentioned sensing means for stopping the overhead conveyormeans.

14. A tray stacking apparatus for use in a recirculating loop bakingtray conveyor system for stacking bak ing trays and the like offerromagnetic material comprising:

said recirculating loop baking tray conveyor system including inlet andoutlet tray conveyor sections each adapted to underlie and support traysat a selected level adjacent a stacking station and having adjacent endsspaced apart a distance substantially greater than the length of a tray,

elevator means at said stacking station for supporting an upright stackof trays, said stack elevator means including laterally spaced endlesschains extending vertically downwardly from adjacent said preselectedlevel at relatively opposite sides of the path of travel of the trays onthe overhead conveyor means, said laterally spaced chains having stacksupports at spaced locations therealong adapted to underlie and supportthe lowermost tray in a stack disposed between the chains, means forsensing the level of the top of the stack of trays between said chains,and elevator drive means responsive to said stack level sensing meansfor driving said chains in a direction to lower the stack until the toptray in the stack is at a level slightly below said selected level,

said baking tray conveyor system including an endless type overheadconveyor means having a lower run spaced above said selected level adistance approximating the height of a tray and extending from said onethe adjacent end of the inlet tray conveyor section over said elevatormeans to said outlet tray conveyor section, magnetic means cooperablewith said lower run of said overhead conveyor means for magneticallyholding the top of a ferromagnetic tray against the lower run foradvance therewith,

means mounting said overhead conveyor means for vertical adjustmentrelative to said inlet and outlet conveyor sections and to said stackelevator means to position the overhead conveyor means at a level abovesaid selected level of said inlet and outlet tray conveyor sections adistance corresponding to the height of the baking trays to allow thetrays to move from the inlet conveyor section along the lower run of theoverhead conveyor and onto the outlet conveyor section for recirculatingthe trays,

a tray stop mounted for movement between an operative position in a pathof movement of the trays on said overhead conveyor and an inoperativeposition out of the path of movement of the trays on the overheadconveyor, said stop means in said operative position thereof beingarranged to stop trays on the overhead conveyor above said elevatormeans,

and tray ejector means, and means operable when a tray is advanced onthe lower run of said overhead conveyor means to a position over a stackon said elevator means and against said tray stop for oper ating saidtray ejector means to discharge the tray from the overhead conveyormeans onto the stack. 15. A tray stacking apparatus according to claim14 wherein said tray ejector means includes an elongated shoe engageablewith the top of a tray on said overhead conveyor means along at least amajor portion of the length of a tray, and means for vertically movingsaid shoe to eject a tray from the lower run of said overhead conveyormeans.

16. A tray stacking apparatus according to claim 14 including meansresponsive to a back-up of trays on the outlet tray conveyor means formoving said stop means to its operative position.

17. A tray unstacking apparatus for use in a recirculating loop bakingtray conveyor system for unstacking baking trays and the like offerromagnetic material comprising:

said recirculating loop baking tray conveyor system including inlet andoutlet tray conveyor sections adapted to underlie and support the traysata selected level adjacent an unstacking station and having adjacentends spaced apart a distance greater than the length of a tray, elevatormeans at said stacking station for supporting an upright stack of trays,said stack elevator means including laterally spaced endless chainsextending vertically downwardly from adjacent said preselected level atrelatively opposite sides of the path of travel of the trays on theoverhead conveyor means, said laterally spaced chains having stacksupports at spaced locations therealong adapted to underlie and supportthe lowermost tray in a stack disposed between the chains, means forsensing the level of the top of the stack of trays between said chains,and elevator drive means responsive to said stack level sensing meansfor driving said chains to raise the stack until the top tray in thestack is at a level slightly below said selected level,

said baking tray conveyor system including an endless type overheadconveyor means having a lower run overlying said elevator means andextending from said inlet conveyor section to said outlet tray conveyorsection and spaced thereabove a distance approximating the height of atray, magnetic means eooperable with said lower run of said overheadconveyor means for magnetically holding the top of a ferromagnetic trayagainst the lower run for advance herewith,

ReJZSgS l means mounting said overhead conveyor means for verticaladjustment relative to said inletand outlet conveyor sections and tosaid stack elevator means to position the overhead conveyor means at alevel above said selected level of said inlet and outlet tray conveyorsections a distance corresponding to the height of the baking trays toallow the trays to move from the inlet conveyor section along the lowerrun of the overhead conveyor and onto the outlet conveyor section forrecirculating the-trays,

tray transfer means including a magnetic pickup head mounted formovement between a lower position engaging the top tray in the stack anda raised position adjacent said lower run of the overhead conveyor meansto transfer a tray from the stack to the overhead conveyor means,

tray sensing means for sensing the presence of a tray on said overheadconveyor means in a zone between a location substantially in advance ofthe elevator means to a location beyond the outlet side of said elevatormeans, drive means operable to drive said pickup head from said raisedposition to said lower position and back, and means responsive to saidtray sensing means for controlling said drive means to prevent operationof said tray transfer means when a tray is in said zone.

18. A tray unstacking apparatus according to claim 17 including meansfor sensing a back-up of trays on said outlet tray conveyor section, andmeans responsive to said last-mentioned means for stopping said overheadconveyor means.

19. A tray unstacking apparatus according to claim 17 including, stackconveyor means operable to advance a stack into position between saidlaterally spaced endless chains of said elevator means, means responsiveto depletion of the stack on the stack supports to a preselected minimumfor operating said stack conveyor means to advance a stack to saidelevator means, and means operative when the stack on the ele vatormeans is exhausted for operating the elevator means to pick up asucceeding stack from the stack conveyor means.

20. An apparatusfor use in a recirculating loop baking tray conveyorsystem for transferring baking trays and the like of ferromagneticmaterial between a tray conveyor system and a stack comprising:

said recirculating loop baking tray conveyor system including inlet andoutlet tray conveyor sections each adapted to underlie and supportbaking trays at a selected level and having adjacent ends spaced apart adistance substantially greater than the length of a tray,

stack elevator means for supporting an upright stack of trays betweensaid adjacent ends of said inlet and outlet tray conveyor sections, saidstack elevator means including laterally spaced endless chains extendingvertically downwardly from adjacent said preselected level at relativelyopposite sides of the path of travel of the trays on the overheadconveyor means, said laterally spaced chains having stack supports atspaced locations therealong adapted to underlie and support thelowermost tray in a stack disposed between the chains, means for sensingthe level of the top of the stack of trays between said chains, andelevator drive means responsive to said stack level sensing means fordriving said chains to position the top tray in the stack at a levelslightly below said selected level of said inlet and outlet trayconveyor sections,

said baking tray conveyor system including an endless type overheadconveyor means having a generally horizontal lower run dimensioned to atleast span the space between the inlet and outlet conveyor sections andspaced above the selected level of the inlet and outlet tray conveyorsections a distance corresponding to the height of the baking trays toallow trays to move from the inlet conveyor section along the lower runof the overhead conveyor and on to the outlet section for recirculatingthe trays, magnetic means cooperable withsaid lower run for magneticallyholding the top of theferromagnetic baking tray against the lower runfor movement therewith,

tray stop means mounted for movement between an operative position inthe path of movement of trays along said overhead conveyor means and aninoperative position out of the path of movement of said trays,

tray transfer means operable to transfer trays between the overheadconveyor means and the stack, means operative when a tray engages saidtray stop means for operating said transfer means to move a tray fromthe overhead conveyor means to a stack on said elevator means,

means for sensing a back-up of trays at a location along said trayconveyor system, and means responsive to said last-mentioned meansformoving said tray stop means from said inoperative position to saidoperative position when a back-up of trays is sensed.

21. An apparatus according to claim 20 including means for sensing thepresence of a tray in the path of movement of said tray stop means andfor preventing movement of said tray stop means to its operativeposition until said overhead conveyor means advances the tray out of thepath of movement of said tray stop means.

22. An apparatusfor use in a recirculating loop baking tray conveyorsystem for transferring baking trays and the like of ferromagneticmaterial between a tray conveyor system and a stack comprising:

said recirculating loop baking tray conveyor system including inlet andoutlet tray conveyor sections each adapted to underlie and supportbaking trays at a selected level and having adjacent ends spaced apart adistance substantially greater than the length of a tray,

stack elevator means for supporting an upright stack of trays betweensaid adjacent ends of said inlet and outlet tray conveyor sections, saidstack elevator means including laterally spaced endless chains extendingvertically downwardly from adjacent said preselected level at relativelyopposite sides of the path of travel of the trays on the overheadconveyor means, said laterally spaced chains having stack supports atspaced locations therealong adapted to underlie and support thelowermost tray in a stack disposed between the chains, rneansjor sensingthe level of the top of the stack of trays between said chains, andelevator drive means responsive to said stack level sensing means fordriving said chains to position the top tray in the stack at a levelslightly below said selected level of said inlet and outlet trayconveyor sections,

said baking tray conveyor system including an endless type overheadconveyor means having a generally horizontal lower run dimensioned to atleast span the space between the inlet and outlet conveyor sections andspaced above the selected level of the inlet and outlet tray conveyorsections a distance corresponding to the height of the baking trays toallow trays to means to transfer a tray from the stack to the overheadconveyor means, and transfer drive means operable to drive said pickuphead from said raised position to said lower position and back,

move from the inlet conveyor section along the lower tray sensing meansfor sensing the presence or absence run of the overhead conveyor andonto the outlet secof a tray on said overhead conveyor means, and tionfor recirculating the trays, magnetic means (0- means responsive to saidtray sensing meansfor conoperable with said lower run for magneticallyholdtrolling said transfer drive means to prevent operaing the top of aferromagnetic baking tray against the tion of said tray transfer meanswhen a tray is on the lower run for movement therewith, overheadconveyor means above the elevator means. tray transfer means including amagnetic pickup head 23. An apparatus according to claim 22 includingmounted for movement between a lower position en means for sensing aback-up of trays in the tray conveyor gaging the top tray in the stackand a raised position system for stopping the overhead conveyor means.adjacent the lower run of the overhead conveyor

1. An apparatus for use in a recirculating loop baking tray conveyorsystem for transferring baking trays and the like of ferromagneticmaterial between a tray conveyor system and a stack comprising: saidrecirculating loop baking tray conveyor system including inlet andoutlet tray conveyor sections each adapted to underlie and supportbaking trays at a selected level and having adjacent ends spaced apart adistance substantially greater than the length of a tray, stack elevatormeans for supporting an upright stack of trays between said adjacentends of said inlet and outlet tray conveyor sections, said stackelevator means including laterally spaced endless chains extendingvertically downwardly from adjacent said preselected level at relativelyopposite sides of the path of travel of the trays on the overheadconveyor means, said Laterally spaced chains having stack supports atspaced locations therealong adapted to underlie and support thelowermost tray in a stack disposed between the chains, means for sensingthe level of the top of the stack of trays between said chains, andelevator (dirve) drive means responsive to said stack level sensingmeans for driving said chains to position the top tray in the stack at alevel slightly below said selected level of said inlet and outlet trayconveyor sections, said baking tray conveyor system including an endlesstype overhead conveyor means having a generally horizontal lower rundimensioned to at least span the space between the inlet and outletconveyor sections, magnetic means cooperable with said lower run formagnetically holding the top of a ferromagnetic baking tray against thelower run for movement therewith, and means mounting said overheadconveyor means for vertical adjustment relative to said inlet and outletconveyor sections and to said endless chains of said stack elevatormeans to position said overhead conveyor means with its lower run at alevel spaced above the selected level of the inlet and outlet trayconveyor sections a distance corresponding to the height of the bakingtrays to allow trays to move from the inlet conveyor section along thelower run of the overhead conveyor and onto the outlet section forrecirculating the trays, and tray transfer means intermittently operableto transfer trays between the overhead conveyor means and the stack. 2.An apparatus according to claim 1 including means operative in responseto advance to a tray along said overhead conveyor means to a positionabove said elevator means for operating said transfer means to move atray from the overhead conveyor means to a stack on said elevator means.3. An apparatus according to claim 1 including stop means mounted formovement between an operative position in the path of movement of traysalong said overhead conveyor means and an inoperative position out ofthe path of movement of said trays, said stop means in said operativeposition thereof being arranged to stop trays on said overhead conveyormeans above said elevator means.
 4. An apparatus according to claim 3including means operative when a tray engages said stop means foroperating said transfer means to move a tray from the overhead conveyormeans to a stack on said elevator means.
 5. An apparatus according toclaim 3 including selectively operable means for moving said stop meansto its operative position, means for sensing the presence of a tray inthe path of movement of said stop means and for preventing movement ofsaid stop means to its operative position until the tray has cleared thepath of movement of the stop means.
 6. An apparatus according to claim 5wherein said selectively operable means for moving said stop means toits operative position includes means for sensing a back-up of trays ata selected location along the outlet conveyor (means) section in thetray conveyor system.
 7. An apparatus according to claim 1 includingmeans operative in response to advance of a tray along said overheadconveyor means to a position above said elevator means for operatingsaid transfer means to discharge a tray from said overhead conveyor ontoa stack on said elevator means, stack conveyor means adjacent the lowerend of said elevator means for conveying a stack of trays laterally awayfrom the elevator means, and means operative when a preselected numberof trays has accumulated in the stack for operating said elevator meansto lower the stack onto said stack conveyor means.
 8. An (appratus)apparatus according to claim 1 wherein said transfer means includes anelongated shoe engageable with the top of the trays on said overheadconveyor means for disengaging the trays from said overhead conveyormeans.
 9. An apparatus according to claim 1 wherein sAid tray transfermeans includes a magnetic pickup head mounted for movement between alower position engaging a tray at the upper end of the stack and araised position to transfer the top tray from the stack to the overheadconveyor means.
 10. An apparatus according to claim 9 including meansfor sensing the presence or absence of a tray on the overhead conveyormeans and for preventing operation of said transfer means until a trayon the overhead conveyor means has cleared a position above theelevator.
 11. An apparatus according to claim 9 including stack conveyormeans adjacent the lower end of said elevator means for advancing astack of articles to the elevator means, means for securing depletion ofthe stack on the elevator means to a preselected minimum for operatingsaid stack conveyor means to advance a succeeding stack of trays to theelevator means, and means operative when the stack of articles on theelevator means is exhausted for driving the elevator means to pick upsaid succeeding stack of trays.
 12. An apparatus according to claim 11including means for sensing the presence or absence of a tray on saidoverhead conveyor means, and means for preventing operation of said traytransfer means when a tray is on the overhead conveyor means above theelevator means.
 13. An apparatus according to claim 10 including meansfor sensing a back-up of trays on the outlet conveyor means, and meansoperated by said last-mentioned sensing means for stopping the overheadconveyor means.
 14. A tray stacking apparatus for use in a recirculatingloop baking tray conveyor system for stacking baking trays and the likeof ferromagnetic material comprising: said recirculating loop bakingtray conveyor system including inlet and outlet tray conveyor sectionseach adapted to underlie and support trays at a selected level adjacenta stacking station and having adjacent ends spaced apart a distancesubstantially greater than the length of a tray, elevator means at saidstacking station for supporting an upright stack of trays, said stackelevator means including laterally spaced endless chains extendingvertically downwardly from adjacent said preselected level at relativelyopposite sides of the path of travel of the trays on the overheadconveyor means, said laterally spaced chains having stack supports atspaced locations therealong adapted to underlie and support thelowermost tray in a stack disposed between the chains, means for sensingthe level of the top of the stack of trays between said chains, andelevator drive means responsive to said stack level sensing means fordriving said chains in a direction to lower the stack until the top trayin the stack is at a level slightly below said selected level, saidbaking tray conveyor system including an endless type overhead conveyormeans having a lower run spaced above said selected level a distanceapproximating the height of a tray and extending from (said one) theadjacent end of the inlet tray conveyor section over said elevator meansto said outlet tray conveyor section, magnetic means cooperable withsaid lower run of said overhead conveyor means for magnetically holdingthe top of a ferromagnetic tray against the lower run for advancetherewith, means mounting said overhead conveyor means for verticaladjustment relative to said inlet and outlet conveyor sections and tosaid stack elevator means to position the overhead conveyor means at alevel above said selected level of said inlet and outlet tray conveyorsections a distance corresponding to the height of the baking trays toallow the trays to move from the inlet conveyor section along the lowerrun of the overhead conveyor and onto the outlet conveyor section forrecirculating the trays, a tray stop mounted for movement between anoperative position in a path of movement of the trays on said overheadconveyor and an inoperative position out of the path of movement of thetrays on the overhead conveyor, said stop means in said operativeposition thereof being arranged to stop trays on the overhead conveyorabove said elevator means, and tray ejector means, and means operablewhen a tray is advanced on the lower run of said overhead conveyor meansto a position over a stack on said elevator means and against said traystop for operating said tray ejector means to discharge the tray fromthe overhead conveyor means onto the stack.
 15. A tray stackingapparatus according to claim 14 wherein said tray ejector means includesan elongated shoe engageable with the top of a tray on said overheadconveyor means along at least a major portion of the length of a tray,and means for vertically moving said shoe to eject a tray from the lowerrun of said overhead conveyor means.
 16. A tray stacking apparatusaccording to claim 14 including means responsive to a back-up of trayson the outlet tray conveyor means for moving said stop means to itsoperative position.
 17. A tray unstacking apparatus for use in arecirculating loop baking tray conveyor system for unstacking bakingtrays and the like of ferromagnetic material comprising: saidrecirculating loop baking tray conveyor system including inlet andoutlet tray conveyor sections adapted to underlie and support the traysat a selected level adjacent an unstacking station and having adjacentends spaced apart a distance greater than the length of a tray, elevatormeans at said stacking station for supporting an upright stack of trays,said stack elevator means including laterally spaced endless chainsextending vertically downwardly from adjacent said preselected level atrelatively opposite sides of the path of travel of the trays on theoverhead conveyor means, said laterally spaced chains having stacksupports at spaced locations therealong adapted to underlie and supportthe lowermost tray in a stack disposed between the chains, means forsensing the level of the top of the stack of trays between said chains,and elevator drive means responsive to said stack level sensing meansfor driving said chains to raise the stack until the top tray in thestack is at a level slightly below said selected level, said baking trayconveyor system including an endless type overhead conveyor means havinga lower run overlying said elevator means and extending from said inletconveyor section to said outlet tray conveyor section and spacedthereabove a distance approximating the height of a tray, magnetic meanscooperable with said lower run of said overhead conveyor means formagnetically holding the top of a ferromagnetic tray against the lowerrun for advance herewith, means mounting said overhead conveyor meansfor vertical adjustment relative to said inlet and outlet conveyorsections and to said stack elevator means to position the overheadconveyor means at a level above said selected level of said inlet andoutlet tray conveyor sections a distance corresponding to the height ofthe baking trays to allow the trays to move from the inlet conveyorsection along the lower run of the overhead conveyor and onto the outletconveyor section for recirculating the trays, tray transfer meansincluding a magnetic pickup head mounted for movement between a lowerposition engaging the top tray in the stack and a raised positionadjacent said lower run of the overhead conveyor means to transfer atray from the stack to the overhead conveyor means, tray sensing meansfor sensing the presence of a tray on said overhead conveyor means in azone between a location substantially in advance of the elevator meansto a location beyond the outlet side of said elevator means, drive meansoperable to drive said pickup head from said raised position to saidlower position and back, and means responsive to said tray sensing meansfor controlling said drive means to prevent operation of said traytransfer means when a tray is in said zone.
 18. A tray unstackingapparatus according to claim 17 including means for sensing a Back-up oftrays on said outlet tray conveyor section, and means responsive to saidlast-mentioned means for stopping said overhead conveyor means.
 19. Atray unstacking apparatus according to claim 17 including, stackconveyor means operable to advance a stack into position between saidlaterally spaced endless chains of said elevator means, means responsiveto depletion of the stack on the stack supports to a preselected minimumfor operating said stack conveyor means to advance a stack to saidelevator means, and means operative when the stack on the elevator meansis exhausted for operating the elevator means to pick up a succeedingstack from the stack conveyor means.
 20. An apparatus for use in arecirculating loop baking tray conveyor system for transferring bakingtrays and the like of ferromagnetic material between a tray conveyorsystem and a stack comprising: said recirculating loop baking trayconveyor system including inlet and outlet tray conveyor sections eachadapted to underlie and support baking trays at a selected level andhaving adjacent ends spaced apart a distance substantially greater thanthe length of a tray, stack elevator means for supporting an uprightstack of trays between said adjacent ends of said inlet and outlet trayconveyor sections, said stack elevator means including laterally spacedendless chains extending vertically downwardly from adjacent saidpreselected level at relatively opposite sides of the path of travel ofthe trays on the overhead conveyor means, said laterally spaced chainshaving stack supports at spaced locations therealong adapted to underlieand support the lowermost tray in a stack disposed between the chains,means for sensing the level of the top of the stack of trays betweensaid chains, and elevator drive means responsive to said stack levelsensing means for driving said chains to position the top tray in thestack at a level slightly below said selected level of said inlet andoutlet tray conveyor sections, said baking tray conveyor systemincluding an endless type overhead conveyor means having a generallyhorizontal lower run dimensioned to at least span the space between theinlet and outlet conveyor sections and spaced above the selected levelof the inlet and outlet tray conveyor sections a distance correspondingto the height of the baking trays to allow trays to move from the inletconveyor section along the lower run of the overhead conveyor and ontothe outlet section for recirculating the trays, magnetic meanscooperable with said lower run for magnetically holding the top of theferromagnetic baking tray against the lower run for movement therewith,tray stop means mounted for movement between an operative position inthe path of movement of trays along said overhead conveyor means and aninoperative position out of the path of movement of said trays, traytransfer means operable to transfer trays between the overhead conveyormeans and the stack, means operative when a tray engages said tray stopmeans for operating said transfer means to move a tray from the overheadconveyor means to a stack on said elevator means, means for sensing aback-up of trays at a location along said tray conveyor system, andmeans responsive to said lastmentioned means for moving said tray stopmeans from said inoperative position to said operative position when aback-up of trays is sensed.
 21. An apparatus according to claim 20including means for sensing the presence of a tray in the path ofmovement of said tray stop means and for preventing movement of saidtray stop means to its operative position until said overhead conveyormeans advances the tray out of the path of movement of said tray stopmeans.
 22. An apparatus for use in a recirculating loop baking trayconveyor system for transferring baking trays and the like offerromagnetic material between a tray conveyor system and a stackcomprising: said recirculating loop baking tray conveyor systemincluding inlet and outlet tray conveyor sections each adapted tounderlie and support baking trays at a selected level and havingadjacent ends spaced apart a distance substantially greater than thelength of a tray, stack elevator means for supporting an upright stackof trays between said adjacent ends of said inlet and outlet trayconveyor sections, said stack elevator means including laterally spacedendless chains extending vertically downwardly from adjacent saidpreselected level at relatively opposite sides of the path of travel ofthe trays on the overhead conveyor means, said laterally spaced chainshaving stack supports at spaced locations therealong adapted to underlieand support the lowermost tray in a stack disposed between the chains,means for sensing the level of the top of the stack of trays betweensaid chains, and elevator drive means responsive to said stack levelsensing means for driving said chains to position the top tray in thestack at a level slightly below said selected level of said inlet andoutlet tray conveyor sections, said baking tray conveyor systemincluding an endless type overhead conveyor means having a generallyhorizontal lower run dimensioned to at least span the space between theinlet and outlet conveyor sections and spaced above the selected levelof the inlet and outlet tray conveyor sections a distance correspondingto the height of the baking trays to allow trays to move from the inletconveyor section along the lower run of the overhead conveyor and ontothe outlet section for recirculating the trays, magnetic meanscooperable with said lower run for magnetically holding the top of aferromagnetic baking tray against the lower run for movement therewith,tray transfer means including a magnetic pickup head mounted formovement between a lower position engaging the top tray in the stack anda raised position adjacent the lower run of the overhead conveyor meansto transfer a tray from the stack to the overhead conveyor means, andtransfer drive means operable to drive said pickup head from said raisedposition to said lower position and back, tray sensing means for sensingthe presence or absence of a tray on said overhead conveyor means, andmeans responsive to said tray sensing means for controlling saidtransfer drive means to prevent operation of said tray transfer meanswhen a tray is on the overhead conveyor means above the elevator means.23. An apparatus according to claim 22 including means for sensing aback-up of trays in the tray conveyor system for stopping the overheadconveyor means.